Internal stress buildup during polymerization of a crosslinked network is a typical result of polymerization shrinkage. As used herein, internal stress is the stress that exists in a solid when no force is applied. Internal stress buildup may decrease the ultimate mechanical properties of the cured polymer and/or limit its applications. For example, in polymeric coatings and dental materials, internal stress may warp or crack the material or the underlying substrate. In addition, volumetric shrinkage during polymerization and the induced shrinkage stress in dental applications can result in tooth-composite adhesive failure.
Currently, commercial photoactivated dental restorative resins are based on dimethacrylates and the reaction mechanism is through chain-growth free radical polymerization. Existing dimethacrylate systems are popular for fillings and other dental prostheses because of their esthetic merit and “cure-on-command” feature.
The photoactivated restorative materials are often sold in separate syringes or single-dose capsules of different shades. If provided in a syringe, the user dispenses (by pressing a plunger or turning a screw adapted plunger on the syringe) the necessary amount of restorative material from the syringe onto a suitable mixing surface. Then the material is placed directly into the cavity, mold, or location of use. If provided as a single-dose capsule, the capsule is placed into a dispensing device that can dispense the material directly into the cavity, mold, etc. After the restorative material is placed, it is photopolymerized or cured by exposing the restorative material to the appropriate light source. The resulting cured polymer may then be finished or polished as necessary with appropriate tools. Such dental restoratives can be used for direct anterior and posterior restorations, core build-ups, splinting and indirect restorations including inlays, onlays and veneers.